Sea Cucumbers: Holothuroidea

Sea cucumbers are soft-bodied sea animals that have a circle of five to twenty tentacles around the mouth. Some sea cucumbers have a thick, muscular body wall, but others are clear and jellylike. Most sea cucumbers look like thick worms or slugs, but some are U-shaped. Depending on the species, the tentacles are simple, fingerlike, featherlike, or flat and shovel- or shieldlike. The large tube feet of some sea cucumbers make them look prickly. Most sea cucumbers are about 20 inches (51 centimeters) long, although some are only a fraction of an inch (millimeters) long and others are longer than 10 feet (3 meters). Sea cucumbers that live in warm, shallow water usually are bright green, red, orange, or yellow. Most sea cucumbers that live in deep, open water are dark. Those that live in deep water but burrow in the bottom are pale gray to white, although some are clear, light purple, or pink.

The water-circulating system that powers the tube feet of sea cucumbers is made up of a ring canal around the throat and long canals that run from the ring to the rear of the animal. In sea stars and other relatives of sea cucumbers, the water-circulating system exchanges water with the environment through a strainer plate that opens to the outside. In most sea cucumbers, however, the strainer plate is inside the animal and opens into the body cavity.

In almost all sea cucumbers, the skeleton of the body wall is made up of microscopic bonelike parts that look like rods, roses, crosses, buttons, tables, or wheels and anchors, among other shapes. In some sea cucumbers, these parts are large and platelike and make the animal rigid. Another important feature of the skeleton is a hard ring around the throat. This ring is an attachment surface for the muscles that move the mouth tentacles and for the forward ends of the muscles that contract the body lengthwise.